System and Method for Wide Angle Optical Surveillance

ABSTRACT

An imaging system ( 30, 30′ ) and corresponding method has a two-dimensional imaging sensor array ( 32 ) and an associated optical system. The optical system includes at least one optical arrangement ( 34, 36α, 36   b ) defining a field of view ( 38   a   , 38   b ) of given angular dimensions and an optical switching mechanism ( 40 ) for alternately switching an optical axis of the imaging system between two directions ( 42α, 42   b ). The optical switching mechanism and the optical arrangement(s) are deployed such that the imaging sensor array generates images of at least two generally non-overlapping fields of view of equal angular dimensions and with diverging optical axes in fixed spatial relation. Rapid switching between the fields of view allows quasi-continuous monitoring of a larger field of view than would otherwise be possible while maintaining sensitivity to transient events. Also disclosed is an infrared search and tracking system based on such imaging systems.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and system for opticalsurveillance of a wide angle or panoramic field of view.

In certain imaging applications, an extremely wide field of view (120°or more) optical system is required so that a very large two dimensionalregion of the object space may be monitored either continuously orrepeatedly at short intervals. Examples of such application include:full earth surveillance from low altitude space platforms, missilelaunch warning from an airborne platform, and air-borne threat detectionfrom a ground base location or a waterborne vessel.

One approach to monitoring such large fields of view is the use of ascanning linear detector array. Examples of this approach are describedin patent publications U.S. Pat. No. 5,347,391 and EP 1416312 A1.Although such systems offer a cost-efficient solution for scanning largeregions, they suffer from a number of shortcomings. Most notably, ascanning linear detector array by its very nature actually views eachgiven pixel of object space for a very small proportion of each scanningcycle. As a result, there is a significant risk of transient events,such as the brief flash accompanying the launch of a missile, beingmissed between scans of the sensor.

An alternative approach is to use staring imaging sensors to monitor theregion of interest. Examples of systems employing staring imagingsensors include patent publications U.S. Pat. No. 6,410,897B1, U.S. Pat.No. 5,534,697A and U.S. Pat. No. 5,300,780A. In most cases, in order toachieve acceptable resolution and avoid problems caused by opticaldistortion, the field of view of each imaging sensor should be limitedto 40-60°. In order to cover a larger solid-angle field of view, ascanning pattern is typically used, resulting in similar problems asdescribed in the context of linear detector arrays discussed above. Fortruly continuous non-scanned monitoring of a large field-of-view at anacceptable resolution, a large number of imaging sensors deployed withoverlapping fields-of-view would be required, thereby rending the systemvery expensive.

One non-limiting example used to illustrate the present invention isthat of protection of naval platforms (waterborne vessels or ships).Ships are relatively vulnerable to attack by many kinds of missiles,such as sea-skimming missiles and gliding bombs, and successfuldeployment of various countermeasures for their defense is dependentupon early detection of incoming threats. Radar has for decades been thestandard technique for search and tracking of airborne threats for navaland other air defense systems. Radar is problematic, however, since itrequires active transmission of radio pulses which give away thepresence of the vessel carrying the tracking system and may be used as aguide beacon to guide offensive armaments towards the vessel. To avoidthis problem, attempts have been made to develop passive (i.e.,non-transmitting) search and tracking systems based upon opticalsensors, and in particular, infrared search and tracking (IRST) systems.

Naval applications highlight the aforementioned shortcomings of bothscanning and staring systems. Implementation of IRST systems for navalapplications poses particular problems in the trade-off betweensufficient sensitivity and avoidance of false alarms. Air-borne threatdetection requires an extremely wide field of view, typically coveringan azimuth of substantially 360°. During the short time-on-target (or“dwell time”) of a scanning system, the background optical noise ofsolar glint from the moving surface of the water is almostindistinguishable from the heat emission of a head-on incoming missile.As a result, scanning systems tend to suffer from insurmountableproblems of high false alarm rates. If a staring system is used,although the dwell time problem is solved, a different problem ofresolution vis-à-vis cost limitations arises. In order to achievereliable detection of a head-on missile threat at sufficient range to beuseful, an angular pixel resolution of at least two, and preferably atleast four, pixels per mille-radian is required. To achieve thisresolution with conventional imaging arrays of several hundred pixelsdimensions, as many as 40-80 imaging sensors would be required,rendering the system overly expensive.

In other contexts, it has been proposed to use a single imaging sensorwith optical multiplexing to perform more than one imaging function.Examples include the aforementioned U.S. Pat. No. 6,410,897 B1 where amovable mirror is used to switch the optical sensor between a wide fieldof view optical objective and a narrow field of view optical objective.A similar concept of switching between narrow and wide fields of view isalso disclosed in U.S. Pat. No. 5,049,740, U.S. Pat. No. 4,486,662 andU.S. Pat. No. 3,804,976. Another example disclosed in U.S. Pat. No.4,574,197 provides a scanning rotating polygon which offers two fieldsof view used for stereoscopic viewing or for two independently steerableoptical telescopes for display on separate screens. None of thesereferences discloses optical multiplexing to offer two similar fields ofview with different optical axes in fixed spatial relation as a solutionfor a staring surveillance system.

A further limitation of the aforementioned existing systems with opticalmultiplexing is that the optical switching frequency is typicallylimited by the read cycle rate of the sensor, i.e., the period taken toexpose the array to incoming illumination and then read the resultinginformation from an array of capacitors associated with each sensorelement. In order to avoid mixing of the content of the two images, thesensor array is exposed for a first integration time to the firstfield-of-view, the associated capacitors are read (a first read cycle),and then the sensor array is exposed to the second field of view and thecapacitors are again read (a second read cycle). This mode of operationis referred to as “Read Then Integrate (RTI). For surveillanceapplications in which it is desired to detect transient events ofduration similar to or less than the read-cycle of the sensor, thisarrangement is problematic since an event may occur while the otherfield of view is being viewed and may therefore be missed by the sensor.

Finally, in the field of staring sensors with a single field of view,there exists a technique known as “Read While Integrate” (RWI) whichsubstantially avoids dead-time during the output reading process betweenintegration periods of a sensor. This technique is particularly usefulwhen monitoring for transient flash events since it helps to ensure thateven a transient event is picked-up by the sensor. “Read WhileIntegrate” also effectively doubles the rate at which frames can beacquired using an array of light-sensitive sensors that produceelectrical charge when the light that they are sensitive to impinges onthem. The principle of RWI will now be illustrated with reference toFIGS. 1 and 2.

Specifically, FIG. 1 shows one such sensor 10, for example an InSbdetector sensitive to infrared light, coupled alternately to twocapacitors 12 and 14 by a switch 16. Capacitors 12 and 14 in turn arealternately coupled to a readout circuit 18 by a switch 20. When acapacitor 12 or 14 is coupled to sensor 10, the capacitor 12 or 14receives and accumulates (“integrates”) the electrical charge producedby sensor 10 as a consequence of the light impinging on sensor 10. Whena capacitor 12 or 14 is coupled to readout circuit 18, readout circuit18 reads the charge accumulated in the capacitor 12 or 14 and dischargesthe capacitor 12 or 14.

FIG. 2 shows the sequence of integration and readout used in the priorart RWI method to acquire images using an array of sensors 10 coupled torespective capacitors 12 and 14 and respective readout circuits 18 asillustrated in FIG. 1. Time increases from left to right in FIG. 2.During odd-numbered time intervals, capacitors 12 accumulate electricalcharges while readout circuits 18 read the electrical chargesaccumulated in capacitors 14 during the immediately preceding timeintervals. During even-numbered time intervals, capacitors 14 accumulateelectrical charges while readout circuits 18 read the electrical chargesaccumulated in capacitors 12 during the immediately preceding timeintervals. The read cycle, i.e., the period between successive readingsfrom the same capacitor, corresponds to a pair of time intervals. Thediagonal arrows in FIG. 2 show the timing of the flow of accumulatedelectrical charge from the capacitors 12 or 14 to readout circuits 18.Note that FIG. 1 illustrates the settings of switches 16 and 20 duringodd-numbered time intervals.

Although RWI provides an effective solution for substantially continuousmonitoring of an imaging system field of view, it is of limited valuewhere a single imaging sensor is used to switch between two or morefields of view since each field of view would still remain unmonitoredfor at least half the read cycle.

There is therefore a need for a wide field-of-view surveillance systembased upon staring imaging sensors which would employ optical switchingto provide quasi-continuous monitoring of a wide-angle field of viewwhile requiring fewer imaging sensors than would otherwise be requiredfor full field of view coverage. It would also be advantageous toprovide an infrared search and tracking system which would provide aneffective passive alternative to radar for detecting threats toplatforms, such as waterborne vessels.

SUMMARY OF THE INVENTION

The present invention is a method and system for optical surveillance ofa wide angle or panoramic field of view.

According to the present invention there is provided an imaging systemcomprising: (a) a two-dimensional imaging sensor array; and (b) anoptical system including: (i) at least one optical arrangementassociated with the imaging sensor array and defining a field of view ofgiven angular dimensions; and (ii) an optical switching mechanism foralternately switching an optical axis of the imaging system between afirst direction and a second direction, the optical switching mechanismand the at least one optical arrangement being deployed such that theimaging sensor array generates images of at least two substantiallynon-overlapping fields of view of equal angular dimensions, thesubstantially non-overlapping fields of view having diverging opticalaxes in fixed spatial relation.

According to a further feature of the present invention, the opticalswitching mechanism includes an apparatus that is alternatelysubstantially transparent and substantially reflective.

According to a further feature of the present invention, the opticalswitching mechanism includes a rotatable disk including at least onepair of alternating segments, a first segment of each the pair beingtransparent and a second segment of each the pair being reflective.

According to a further feature of the present invention, the at leastone pair includes at least two pairs of segments, wherein thetransparent segments are transparent to non-identical ranges ofwavelengths and the reflective segments are reflective to non-identicalranges of wavelengths.

According to a further feature of the present invention, the opticalswitching mechanism includes a plurality of microelectromechanicalshutters, the apparatus being substantially transparent when theshutters are open and substantially reflective when the shutters areclosed.

According to a further feature of the present invention, the opticalswitching mechanism includes a pair of prisms and a prism displacementmechanism operative to displace at least one of the pair of prisms suchthat the pair of prisms are alternately adjacent and apart, theapparatus being substantially transparent when the prisms are adjacentand substantially reflective when the prisms are apart.

According to a further feature of the present invention, there isprovided an imaging assembly comprising a plurality of theaforementioned imaging systems, wherein the imaging systems are deployedin fixed spatial relation such that the substantially non-overlappingfields of view of the plurality of imaging systems together form asubstantially contiguous effective field of view spanning at least 1200,and preferably at least 180°, and most preferably substantially 360°.

According to a further feature of the present invention, the pluralityof the imaging systems includes at least three of the imaging systems.

According to a further feature of the present invention, the opticalswitching mechanism switches between the fields of view at afield-of-view switching rate, the imaging system further comprising aread arrangement for reading accumulated information from thetwo-dimensional imaging sensor array at a read cycle rate, wherein thefield-of-view switching rate is greater than the read cycle rate.

According to a further feature of the present invention, there are alsoprovided: (a) a first set of read capacitors including a capacitorassociated with each sensor element of the two-dimensional imagingsensor array; (b) a second set of read capacitors including a capacitorassociated with each sensor element of the two-dimensional imagingsensor array; and (c) a sensor switching arrangement associated with thetwo-dimensional imaging sensor array, the first and second sets of readcapacitors and the optical switching mechanism, the sensor switchingarrangement being configured to switch connections from each sensorelement of the two-dimensional imaging sensor array betweencorresponding capacitors of the first and second sets of read capacitorssynchronously with switching of the optical switching mechanism betweenthe two fields of view such that the first set of read capacitorsaccumulate information corresponding to a first of the two fields ofview and the second set of read capacitors accumulate informationcorresponding to a second of the two fields of view.

According to a further feature of the present invention, the sensorswitching arrangement switches the connections at a field-of-viewswitching rate, the imaging system further comprising a read arrangementfor reading accumulated information from the first and second sets ofread capacitors at a read cycle rate, wherein the field-of-viewswitching rate is greater than the read cycle rate.

According to a further feature of the present invention, there are alsoprovided: (a) a third set of read capacitors including a capacitorassociated with each sensor element of the two-dimensional imagingsensor array; and (b) a fourth set of read capacitors including acapacitor associated with each sensor element of the two-dimensionalimaging sensor array, wherein the sensor switching arrangement and theread arrangement are further associated with the third and fourth setsof read capacitors, the sensor switching arrangement and the readarrangement being configured such that: (i) during a first half of theread cycle, the sensor switching arrangement switches connections fromeach sensor element of the two-dimensional imaging sensor array betweencorresponding capacitors of the first and second sets of read capacitorssynchronously with switching of the optical switching mechanism betweenthe two fields of view while the read arrangement reads the third andfourth sets of read capacitors; and (ii) during a second half of theread cycle, the sensor switching arrangement switches connections fromeach sensor element of the two-dimensional imaging sensor array betweencorresponding capacitors of the third and fourth sets of read capacitorssynchronously with switching of the optical switching mechanism betweenthe two fields of view while the read arrangement reads the first andsecond sets of read capacitors.

According to a further feature of the present invention, there is alsoprovided a processor configured for analyzing a sequence of images fromthe imaging system to determine whether they indicate the presence of atransient event.

There is also provided according to the teachings of the presentinvention, an infrared search and tracking (IRST) system for awaterborne vessel, the system comprising: (a) at least one stabilizedplatform; and (b) an imaging assembly deployed on the at least onestabilized platform, the imaging assembly including a plurality of theaforementioned imaging systems, wherein the imaging systems are deployedin fixed spatial relation such that the fields of view of the pluralityof imaging systems together form a substantially contiguous effectivefield of view spanning substantially 360°.

According to a further feature of the present invention, the at leastone stabilized platform is implemented as a plurality of the stabilizedplatforms, and wherein each of the stabilized platforms carries acorresponding subgroup of the plurality of imaging systems, the subgroupproviding fields of view which together form a substantially contiguouseffective field of view spanning a corresponding given angle, thecorresponding given angles together substantially spanning 360°.

According to a further feature of the present invention, the at leastone stabilized platform is implemented as two stabilized platforms fordeployment on opposite sides of the waterborne vessel, and wherein afirst of the stabilized platforms carries a first subgroup of theplurality of imaging systems, the first subgroup providing fields ofview which together form a substantially contiguous effective field ofview spanning at least a first angle, and wherein the second of thestabilized platforms carries a second subgroup of the plurality ofimaging systems, the second subgroup providing fields of view whichtogether form a substantially contiguous effective field of viewspanning at least 360° minus the first angle, the first and secondsubgroups thereby together providing an effective field of viewsubstantially spanning 360°.

According to a further feature of the present invention, the at leastone stabilized platform is stabilized to within a predefined level ofprecision, the system further comprising a fine stabilizationarrangement including: (a) an inertial sensor arrangement associatedwith the stabilized platform for measuring residual motion of thestabilized platform; and (b) a processing system responsive to theinertial sensor arrangement to process images from the imaging assemblyso as to correct the images so as to compensate for the residual motion.

According to a further feature of the present invention, each of thefields of view for each imaging system has a spatial resolution of atleast two pixels per mille-radian.

According to a further feature of the present invention, thesubstantially contiguous effective field of view spans substantially360° in azimuth and spans a range of at least about 8° in elevation.

According to a further feature of the present invention, there is alsoprovided a processing system associated with the imaging assembly andconfigured to process images from the imaging systems according to a setof target detection criteria to identify suspected targets.

According to a further feature of the present invention, the targetdetection criteria include substantially continuous detection of asuspected target for a period in excess of about half a second.

According to a further feature of the present invention, there is alsoprovided a gimbaled narrow field-of-view infrared imaging sensor havinga field of view not greater than about 3°, the gimbaled narrowfield-of-view infrared imaging sensor being associated with theprocessing system for directing towards suspected targets for evaluationof the suspected targets.

According to a further feature of the present invention, the opticalswitching mechanism is configured to switch the optical axis of theimaging system in two non-parallel switching planes such that eachimaging system generates images of at least four fields of view forminga two-dimensional array of fields of view.

There is also provided according to the teachings of the presentinvention, an infrared search and tracking (IRST) system comprising animaging assembly including a plurality of the aforementioned imagingsystems, wherein the imaging systems are deployed in fixed spatialrelation such that the fields of view of the plurality of imagingsystems together form a substantially contiguous effective field of viewspanning at least about 40°.

There is also provided according to the teachings of the presentinvention, a method for acquiring images of two fields of view using asingle two-dimensional imaging sensor array, the method comprising: (a)providing an optical arrangement including an optical switchingmechanism for alternately directing an image of the two fields of viewonto the imaging sensor array; (b) providing for each sensor of theimaging sensor array at least one pair of read capacitors and a sensorswitching arrangement for switching a connection from each of thesensors between the corresponding pair of read capacitors; (c)synchronously operating the optical switching mechanism and the sensorswitching arrangement at a field-of-view switching rate such that afirst of the pair of read capacitors accumulates informationcorresponding to a pixel of a first of the fields of view and a secondof the pair of read capacitors accumulates information corresponding toa pixel of a second of the fields of view; and (d) reading accumulatedinformation from the pairs of read capacitors at a read cycle rate so asto acquire a sequence of images of each of the two fields of view,wherein the field-of-view switching rate is greater than the read cyclerate.

According to a further feature of the present invention, thefield-of-view switching rate is at least four times the read cycle rate.

According to a further feature of the present invention, the sequence ofimages is analyzed to determine whether they indicate the presence of atransient event.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1, described above, illustrates a read-while-integrate sensor ofthe prior art;

FIG. 2, described above, is a timing chart for the prior art sensor ofFIG. 1;

FIG. 3 illustrates a first implementation of an imaging systemconstructed and operative according to the teachings of the presentinvention;

FIG. 4 illustrates a variant implementation of the imaging system ofFIG. 3;

FIGS. 5A and 5B are schematic representations of components of apreferred implementation of a sensor assembly for use in the imagingsystem of FIG. 3 or 4, the components being shown in first and secondstates, respectively;

FIG. 6 is a timing chart for the sensor assembly of FIGS. 5A and 5B;

FIGS. 7A and 7B illustrate two implementations of a rotating disk foruse in an optical switching arrangement in the imaging system of FIG. 3or 4;

FIGS. 8A and 8B illustrate schematically an alternative implementationof an optical switching arrangement based upon relative displacement ofa pair of prisms, shown in a transmitting state and a reflecting state,respectively;

FIGS. 9A and 9B illustrate schematically an alternative implementationof an optical switching arrangement employing micro-electromechanicalshutters shown in an open and closed state, respectively;

FIG. 10 shows three of the imaging systems of FIG. 4 deployed formonitoring a full 360 degrees of azimuth;

FIG. 11A illustrates schematically a further variant of the imagingsystem of FIG. 4 for monitoring three fields of view;

FIG. 11B illustrated schematically an further variant of the imagingsystem of FIG. 4 for monitoring four fields of view in a two-dimensionalarray layout;

FIGS. 12A and 12B are schematic illustrations of a ship fitted with anIRST system based on a plurality of the imaging arrangements of FIG. 4,constructed and operative according to the teachings of the presentinvention, illustrating two different options of deployment of thesensors of the system;

FIG. 13 is a schematic representation of the main components of the IRSTsystem of the present invention;

FIG. 14 is a block diagram of the IRST system of the present invention;

FIG. 15 is a schematic representation of a dual field-of-view imagingsystem for use in the IRST system of the present invention;

FIG. 16 is a block diagram of the dual field-of-view imaging system ofFIG. 15; and

FIG. 17 is a flow diagram illustrating a sequence of processing ofimages from the imaging sensors of the present invention according to aset of target detection criteria.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method and system for optical surveillance ofa wide angle or panoramic field of view.

The principles and operation of methods and systems according to thepresent invention will be better understood with reference to thedrawings and the accompanying description.

Turning now to the drawings, FIGS. 3-11B illustrate various preferredembodiments of an imaging system, generally designated 30, 30′,constructed and operative according to the teachings of the presentinvention. FIGS. 12-17, described below, will then present aparticularly preferred but non-limiting exemplary implementation of thepresent invention as part of an infrared search and track (IRST) systemfor naval platforms. Referring first particularly to FIGS. 3 and 4,generally speaking, imaging system 30, 30′ includes a two-dimensionalimaging sensor array 32 and an optical system including: at least oneoptical arrangement 34, 36 a, 36 b defining a field of view 38 a, 38 bof given angular dimensions; and an optical switching mechanism 40.Optical switching mechanism 40 is configured to alternately switch anoptical axis of the imaging system between a first direction 42 a and asecond direction 42 b. Optical switching mechanism 40 and the at leastone optical arrangement 34, 36 a, 36 b are deployed such that imagingsensor array 32 generates images of at least two substantiallynon-overlapping fields of view 38 a, 38 b of equal angular dimensionswith diverging optical axes in fixed spatial relation.

At this stage, it will already be appreciated that the present inventionoffers profound advantages of economy in cost and size by employing asingle imaging sensor array with optical switching for monitoring doublethe non-switched field of view without any loss of spatial resolution.Thus, if an imaging system with its associated optics is configured toprovide a basic field of view spanning 60°, the present invention may beemployed to provide images spanning 120° using the same single opticalsensor and without loss of spatial resolution. This principle can beextended by employing more than one optical switching mechanism 40 tomonitor three or more fields of view using a single sensor, asillustrated schematically in FIGS. 11A and 11B. Furthermore, a smallgroup of imaging systems may be used together to span wide angle or evenpanoramic views, as illustrated schematically in FIG. 10. These andother advantages of the present invention will become clearer from thefollowing description.

Before addressing the features of the present invention in detail, itwill be useful to define certain terminology as used herein in thedescription and claims. Firstly, reference is made to fields of viewwhich are “substantially non-overlapping”. This phrase is used to referto fields of view which primarily cover different regions, but does notexclude the possibility of slight overlap at the periphery of theregions. To the contrary, it is typically preferred that the differentfields of view have slight overlap (typically less than 5%) in order toensure complete coverage of the combined region. Fields of view withperipheral overlap of this type are within the scope of “substantiallynon-overlapping” as used herein.

The term “two-dimensional imaging sensor array” is used herein to referto any and all types of two-dimensional imaging sensor array, or“focal-plane array”. The type of sensor used, and the design of theassociated optical components, is chosen according to the wavelengths ofradiation to be sensed, as is well known to those ordinarily skilled inthe art. Thus, the imaging sensor array may be a visible ornear-infrared sensor chip of CCD or CMOS type, an ultraviolet sensorchip, or a far-infrared sensor of any type. For the preferredapplications of the present invention for detection of missile threatsor the like, infrared sensor arrays sensitive to radiation in themicron-range, and most preferably, an imaging sensor based on InSbdetectors sensitive to infrared radiation of wavelengths between 3 and 5microns.

It should be noted that the illustrations of read circuitry in FIGS. 1,5A and 5B are schematic for the purpose of illustrating the operatingprinciples of the read circuitry. Clearly, the specific Implementationof the circuitry depends upon the type of sensor used, as will be clearto one ordinarily skilled in the art.

Turning now to the features of imaging system 30 in more detail, opticalswitching mechanism 40 typically includes an apparatus that isalternately substantially transparent and substantially reflective.Preferably, a high speed switching arrangement is used, capable ofswitching between its two states with a transition of less than onemillisecond. One or more additional reflective element (e.g., a mirror)is typically used as shown to ensure the desired geometrical relationbetween the fields of view, as will be clearly understood.

In the preferred implementations of FIGS. 3 and 4, optical switchingmechanism 40 is based upon a rotatable disk 44 including at least onepair of alternating segments, a first segment 46 of each pair beingtransparent and a second segment 48 of each pair being reflective. Atwo-segment implementation of disk 44 is illustrated in FIG. 7A. Thedisk is preferably driven by a synchronous electric motor.

FIG. 7B shows a further preferred implementation in which at least onepair of segments 46 a and 48 a are selectively transparent andreflective, respectively, in a selected wavelength band. The possibilityof adding selective filtering provides an additional option for reducingfalse alarm rates (FAR) for flash detection. Specifically, it is knownthat a flash event such as launch of a missile produces an intensitypeak at wavelengths between 4.0 and 4.8 microns. Thus, in the preferredcase of an imaging sensor of InSb detectors sensitive to infraredradiation of wavelengths between 3 and 5 microns, segment 46 a isimplemented as a bandpass filter with a passband between 4.5 and 4.8microns and 46 b is implemented as an absorptive bandpass filter thatpasses only wavelengths below 4 microns and absorbs other wavelengths.Similarly, segment 48 a is implemented as a dichroic mirror thatreflects only wavelengths between 4.5 and 4.8 microns while segment 48 bis implemented as a dichroic mirror that reflects only wavelengths below4 microns and absorbs other wavelengths. In this way, each revolution ofthe disk generates two image exposures of each FOV in differentwavelength bands. By comparing the resulting images, it is possible todistinguish reliably between genuine “flash” events which have a largedifferential between the two bands and other transient intensity peakssuch as the glint of sunlight reflected from a moving surface which tendto be relatively evenly dispersed between the two wavelength bands.

Turning now to FIGS. 8A and 8B, there is shown schematically analternative implementation of optical switching mechanism 40 including apair of prisms 50, 52 and a prism displacement mechanism (representedhere schematically by arrow 54) operative to displace at least one ofprisms 50, 52 such that prisms 50, 52 are alternately adjacent (as inFIG. 8A) to provide a substantially transparent state in which totalinternal reflection is frustrated and apart (as in FIG. 8B) to provide asubstantially reflective state by total internal reflection.

Turning now to FIGS. 9A and 9B, there is shown schematically analternative implementation of optical switching mechanism 40 including aplurality of reflective micro-electromechanical shutters 56. In thiscase, the mechanism is substantially transparent when shutters 56 areopen (FIG. 9A) and substantially reflective when shutters 56 are closed(FIG. 9B). Such a mechanism can be implemented using, for example, thetechnology described in PCT publication no. WO02/13168. Suitablemicro-electromechanical shutter elements are commercially available fromFlixel Ltd., (Tel Aviv, Israel).

Referring briefly specifically to FIG. 4, it should be noted that thisis similar to the implementation of FIG. 3 except for the use of anoptical arrangement to reduce the size of a footprint of the opticalbundle passing through optical switching mechanism 40. This is typicallyachieved by designing the optics to provide an intermediate image planeat or near optical switching mechanism 40. This allows the opticalswitching mechanism to be more compact than would otherwise be possible.In all other respects, the implementation of FIG. 4 is fully analogousin structure and operation to that of FIG. 3.

Turning now to the remaining features of imaging system 30, theseinclude a sensor read arrangement 60 for reading data (typicallyaccumulated charge) from sensor elements of the two-dimensional imagingsensor array and a controller 62 for controlling synchronous operationof the optical switching mechanism and the read arrangement to ensurecorrect separation of data from the two fields of view. Data from readarrangement 60 is transferred to a processing system 214 for subsequentimage processing, preferably analyzing a sequence of images from theimaging system to determine whether they indicate the presence of atransient event. One example of the operation of processing system 214will be addressed further below.

As mentioned above, switching between different fields of view maypresent problems for detection of flash events (e.g. a missile launch)of duration similar to or shorter than the period of the sensor readcycle. Specifically, there is a risk that the sensor may be viewing adifferent field of view for the entire duration of the flash event. Itis a particular feature of certain most preferred embodiments of thepresent invention that switching between the plural fields of viewmonitored by the imaging sensor array is performed at a frequencygreater than, and preferably at least four times greater than, the readcycle rate. This requires provision of a specially configured sensorread arrangement 60 as will now be described with reference to FIGS.5A-6.

Specifically, with reference to FIGS. 5A and 5B, sensor read arrangement60 includes a first set of read capacitors including a capacitor 112associated with each sensor element 64 of the two-dimensional imagingsensor array and a second set of read capacitors including a capacitor114 associated with each sensor element 64 of the two-dimensionalimaging sensor array. A sensor switching arrangement 66 is configuredperform fast switching (i.e. at a rate greater than the read cycle rate)of connections from each sensor element 64 between correspondingcapacitors 112, 114, synchronously with switching of the opticalswitching mechanism between the two fields of view, such that the firstset of read capacitors 112 accumulate information corresponding to thefirst field of view 38 a and the second set of read capacitors 114accumulate information corresponding to the second field of view 38 b.Thus, the integration time for the image of each FOV is made up of asummation of short exposures interspersed during a single read cycle ofthe imaging array. In this manner, the revisit delay during which atransient event in one FOV could be missed can be reduced to very muchshorter than the read cycle period. The result is pseudo-continuousobservation in the two FOVs while maintaining the overall sensitivity ofthe sensor system.

Although a basic implementation of this fast-switching methodologyrequires only two read capacitors per sensor element, as described up tothis point, most preferred implementations employ four read capacitorsper sensor element of the array to combine the fast switchingmethodology with the read-while-integrate approach described above.Thus, in the case illustrated here, capacitors 112 and 114 aresupplemented by third and fourth sets of read capacitors 113 and 115.During a first time period as illustrated in FIG. 5A, fastoptical/electrical switching is performed as described above toaccumulate image data for the two FOVs in (sets of) capacitors 112 and114 while the previous images of both FOVs are read from capacitors 113and 115, either simultaneously or sequentially. Then, during a secondhalf of the read-cycle, switching arrangement 66 performs fast switchingbetween capacitors 113 and 115 while capacitors 112 and 114 are read, asillustrated in FIG. 5B. In this way, the revisit delay for each field ofview can be further reduced.

Parenthetically, it should be appreciated that the choice of four readcapacitors per sensor element is sufficient for the applicationdescribed, but may be increased as necessary for alternativeimplementations according to the present invention. Thus, for example,the band filter implementation of FIG. 7B would optimally be implementedwith eight read capacitors per sensor element in order to record allfour images within a read-while-integrate framework. Similarly, animaging sensor with two optical switching mechanisms used to generatethree distinct FOVs as illustrated in FIG. 11A would require six readcapacitors per sensor element while that of FIG. 11B which generatesfour distinct FOVs would require eight read capacitors per sensorelement.

It will also be clear to one ordinarily skilled in the art that thespecific topography of the switching arrangements described in thisexample can be rearranged without departing from the principles of thepresent invention.

It will be appreciated that the principles of the present invention maybe applied to a wide range of applications wherever it is useful tomonitor a plurality of fields of view using a reduced number of staringsensors. Although applications employing a single imaging system fallwithin the scope of the present invention, most preferredimplementations provide an imaging assembly where two or more imagingsystems according to the present invention are deployed in fixed spatialrelation such that the substantially non-overlapping fields of view ofthe plurality of imaging systems together form a substantiallycontiguous effective field of view spanning at least 120°, and morepreferably at least 180°. For various surveillance applications, threeor more field-switching imaging systems are employed to providepanoramic (360° azimuth) coverage. In this context, it will be notedthat the plural FOVs of each individual imaging system need not becontiguous if the FOVs of the different imaging systems are interspersedin a complementary manner so as to together offer substantiallycontiguous coverage.

As mentioned earlier, the scope of the present invention is not confinedto FOV doubling, but includes, in general, M-fold FOV expansion. FIG.11A illustrates FOV tripling according to the present invention. FIG. 1shows imaging system 30 including two optical switching mechanisms 144and 244 that are similar to optical switching mechanism 40 describedabove. For illustrational simplicity, the associated mirrors for foldingthe reflected optical paths 142 and 242 of the imaging system are notshown. When both mechanisms 144 and 244 are in their transparent state,the optical path of system 30 is the straight-ahead optical path of theimaging system 30. When optical switching mechanism 144 is in itsreflective state, the optical path of system 30 is reflected opticalpath 142 corresponding to a reflected FOV to the left of thestraight-ahead optical path 140 of FIG. 11A. When optical switchingmechanism 144 is in its transparent state and optical switchingmechanism 244 is in its reflective state, the optical path of the systemis reflected optical path 242 corresponding to a reflected FOV to theright of the straight-ahead optical path 140 of FIG. 11A. FIG. 11Billustrates a further variation where two switching mechanisms are usedin series with both optical axes of the first switching mechanism beingfurther switched by a second switching mechanism to achieve four fieldsof view monitored by a single imaging arrangement. In the caseillustrated here, the switching mechanisms operate in non-parallelplanes, and preferably orthogonally, to provide a 2×2 array of fields ofview 38 a-38 d. This option is of particular importance where theinherent elevational field of view of each sensor is too small for agiven application.

Turning now to FIG. 12A-17, these illustrate a schematically an infraredsearch and tracking (IRST) system, generally designated 202, constructedand operative according to the teachings of the present invention andbased upon imaging arrangements as described hereinabove. The coverageafforded to a ship 200 by two different configurations of the IRSTsystem 202 are illustrated in FIGS. 12A and 12B, respectively. Thecomponents making up IRST system 202 are shown in FIGS. 13 and 14. Ingeneral terms, IRST system 202 includes at least one, and preferablytwo, stabilized platforms 204 upon which are deployed an imagingassembly 206 which includes a plurality of imaging systems 208. As bestseen in FIGS. 15 and 16, each imaging system 208 is a non-limitingpreferred implementation of imaging arrangement 30 or 30′ describedabove, including two-dimensional imaging sensor array 32 and an opticalsystem including: optical arrangements 34, 36 a, 36 b defining fields ofview 38 a, 38 b; and optical switching mechanism 40 configured toalternately switch the optical axis of the imaging system between firstdirection 42 a and second direction 42 b. The plurality of imagingsystems 208 are deployed in known spatial relation such that the fieldsof view of the plurality of imaging systems together form asubstantially contiguous effective field of view spanning substantially360°, as illustrated schematically in FIGS. 12A and 12B.

As before, it will be appreciated that the present invention offersprofound advantages of economy in cost and size for implementation of anIRST system by multiplying the field of view which can be monitored byeach imaging sensor array without any loss of spatial resolution. Thesystem is effectively a staring system, thereby avoiding all of theaforementioned limitations of scanning systems. Thus, if an imagingsystem with its associated optics is configured to provide a basic fieldof view spanning 15°, complete coverage of a panoramic (360° azimuth)field of view can be provided by about 12 imaging systems in contrast tothe 24 sensors which would be required according to the teachings of theprior art. This number can be reduced further if multiple switching isemployed to provide more than two fields of view per imaging sensor. Thesystem also preferably provides an elevation field of view of at leastabout 8°, in contrast to the 20-40 of conventional scanning systems,thereby providing effective detection of air-launched missiles andgliding bombs. Furthermore, vertical switching of fields of view mayadditionally be used to increase the elevation field of view. Forexample, an imaging sensor with a basic FOV of 8°×8° may be used withhorizontal and vertical switching in sequence to monitor four quadrantscovering a total of roughly a 16°×16° field of view as shown in FIG.11B.

Turning now to the features of IRST system 202 in more detail, it willbe noted that the subdivision of the imaging systems between one or morestabilized platform is somewhat arbitrary. In certain cases where asuitable panoramic vantage point is available, such as high on a mast ofa ship, it is possible to mount all of the required imaging systems on acommon platform. In many cases, however, it is convenient for practicalreasons to subdivide the imaging systems around the periphery of thevessel. In the case of FIG. 12A, this subdivision is shown between leftand right of the vessel. The natural choice for such an arrangement isto subdivide the systems symmetrically such that each covers just over180°, thereby together offering 360° coverage. In the case of FIG. 12B,a front/rear subdivision is used. Here, the subdivision may or may notbe symmetrical. Although illustrated here in cases where two stabilizedplatforms are used, it will be clearly appreciated that there is no reallimit to the number of platforms to be used and further subdivision ofthe imaging systems is possible.

In order to provide the required sensitivity for detecting distanthead-on missiles, each of the fields of view for each imaging systempreferably has a spatial resolution of at least two, and preferably atleast four, pixels per mille-radian. Each imaging system field of viewpreferably spans a range of at least about 8° in elevation.

As already mentioned, the imaging systems of IRST system 202 are mountedon one or more stabilized platform 204. Such platforms, well known inthe art and commercially available from numerous sources, typicallyprovide isolation from gross motions of pitch and roll of a ship, but donot compensate motions sufficiently to completely eliminate their impacton high resolution images such as those of the IRST system 202.Accordingly, most preferred implementations of the present inventionprovide a fine stabilization arrangement (see FIG. 14) including aninertial sensor arrangement 210 associated with stabilized platform 204for measuring residual motion of the stabilized platform, and astabilization module 212 responsive to inertial sensor arrangement 210to process images from the imaging assembly so as to correct the imagesso as to compensate for the residual motion. Stabilization module 212 ispreferably implemented as part of a main processing system 214 whichperforms all of the functions of image processing, analysis, targetdetection and tracking for the IRST system 202. Preferably, inertialsensor arrangement 210 provides output indicative of platform motionwith a resolution better than 1 mille-radian, and more preferably, ofthe order of 0.1 mille-radian. This allows stabilization of imagesprocessed at sub-pixel resolution, thereby greatly increasing thereliability of “point” target detection.

One of the functions preferably performed by processing system 214 isprocessing images from the imaging systems according to a set of targetdetection criteria to identify suspected targets. Of particularimportance is a detection criterion requiring substantially continuousdetection of a suspected target for a period in excess of about half asecond. This criterion is highly effective for eliminating false alarmsdue to background optical noise, and particularly solar glint from thesurface of the sea.

Turning now to FIG. 17, this illustrates in more detail a non-limitingpreferred implementation of the target detection algorithms implementedby the system of the present invention. Specifically, after initialpreprocessing such as application of a non-uniformity correction (NUC)and combining of the separate sensor images into a mosaic (step 220),the scene is preferably segmented into regions (step 222), such as“sea”, “sky” and “land”, each of which tends to have differentbackground noise characteristics and therefore benefits from independentprocessing. Segmentation techniques suitable for segmenting the sceneaccording to various known characteristics are well known in the art andwill not be addressed here in detail. The segments of the image are thenprocessed by various contrast enhancement techniques (step 224), chosento be appropriate for each type of background, so as to facilitatetarget detection. One such technique which may be useful is referred toas “whitening” which removes all components other than the high spatialfrequencies which are relevant for target detection. The technique isreferred to as “whitening” because the remaining picture is largely voidof perceivable image data, and is primarily the “white noise” of thesensed image together with the data relevant for target sensing. Theprocessing then proceeds with false alarm rejection 226 which implementsvarious false alarm rejection algorithms, preferably including but notlimited to the rejection of suspected targets which do not persist for arequisite minimum time period, preferably of at least about half asecond. The processing then continues with frame-to-frame tracking ofsuspected targets (step 228) to verify that the spatial and temporalvariations of the suspected target correspond to possible scenarios ofbehavior of a real target, and to maintain position tracking of thetarget. Finally, the image of the target is checked against variouscriteria and/or look-up tables of characteristic targets in an attemptto classify the tracked object, for example, as a verified target,verified non-threatening object or an unclassified potential threat.According to the result of this classification, an event requiringfurther action (automatic, semi-automatic or manual) may be declared.

According to one preferred option, IRST system 202 also includes agimbaled narrow field-of-view infrared imaging sensor 216 (FIGS. 13 and14) having a field of view not greater than about 3° which is used forfurther action after detection of a threat or potential threat. Gimbalednarrow field-of-view infrared imaging sensor 216 is preferablyassociated with processing system 214 for directing towards suspectedtargets for evaluation of the suspected targets. This providesadditional higher resolution information for automated classification ofa target and/or offers a human operator a visual display of thesuspected target for evaluation. The narrow field-of-view infraredimaging sensor 216 is preferably gimbaled to provide a field of regard(FOR) giving 360° azimuth coverage and a range of elevations greaterthan the coverage of imaging assembly 206. Most preferably, the gimbalprovides high ranges of elevation (spanning at least 60°, and mostpreferably 90-100°) to offer roughly hemispherical coverage forcontinuous tracking of a detected target outside the field of view ofimaging assembly 206.

In all other respects, it should be appreciated that imaging systems 208are preferably implemented according to any or all features describedabove in the context of imaging systems 30 and 30′. For example, IRSTsystem 202 is preferably configured for detecting both slowly varyingtargets, such as incoming missiles in flight, i.e., with a time constantgreater than that of the background solar clutter, and transient “flash”events, such as the launching of a missile, with a time constant lessthan that of the background solar clutter. For this purpose, the imagingsystem preferably includes fast switching and other features describedabove which are helpful in detection of transient flash events.

Although the above illustration has been given in the context ofapplication to a navel vessel, it will be appreciated that the presentinvention may equally be applied to other IRST systems or other types ofsurveillance systems from other land-based, airborne or satelliteplatforms. Details of such implementations, and modifications wherenecessary, will all be clear to one ordinarily skilled in the art byanalogy to the above description.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made withinthe scope of the invention as defined by the appended claims.

1. An imaging system comprising: (a) a two-dimensional imaging sensor array; and (b) an optical system including: (i) at least one optical arrangement associated with the imaging sensor array and defining a field of view of given angular dimensions; and (ii) an optical switching mechanism for alternately switching an optical axis of the imaging system between a first direction and a second direction,  said optical switching mechanism and said at least one optical arrangement being deployed such that said imaging sensor array generates images of at least two substantially non-overlapping fields of view of equal angular dimensions, said substantially non-overlapping fields of view having diverging optical axes in fixed spatial relation.
 2. The imaging system of claim 1, wherein said optical switching mechanism includes an apparatus that is alternately substantially transparent and substantially reflective.
 3. The imaging system of claim 1, wherein said optical switching mechanism includes a rotatable disk including at least one pair of alternating segments, a first segment of each said pair being transparent and a second segment of each said pair being reflective.
 4. The imaging system of claim 3, wherein said at least one pair includes at least two pairs of segments, wherein said transparent segments are transparent to non-identical ranges of wavelengths and said reflective segments are reflective to non-identical ranges of wavelengths.
 5. The imaging system of claim 1, wherein said optical switching mechanism includes a plurality of microelectromechanical shutters, said apparatus being substantially transparent when said shutters are open and substantially reflective when said shutters are closed.
 6. The imaging system of claim 1, wherein said optical switching mechanism includes a pair of prisms and a prism displacement mechanism operative to displace at least one of said pair of prisms such that said pair of prisms are alternately adjacent and apart, said apparatus being substantially transparent when said prisms are adjacent and substantially reflective when said prisms are apart.
 7. An imaging assembly comprising a plurality of imaging systems according to claim 1, wherein said imaging systems are deployed in fixed spatial relation such that said substantially non-overlapping fields of view of said plurality of imaging systems together form a substantially contiguous effective field of view spanning at least 120°.
 8. The imaging assembly of claim 7, wherein said substantially contiguous effective field of view spans at least 180°.
 9. The imaging assembly of claim 7, wherein said substantially contiguous effective field of view spans 360°.
 10. The imaging assembly of claim 7, wherein said plurality of said imaging systems includes at least three of said imaging systems.
 11. The imaging system of claim 1, wherein said optical switching mechanism switches between said fields of view at a field-of-view switching rate, the imaging system further comprising a read arrangement for reading accumulated information from said two-dimensional imaging sensor array at a read cycle rate, wherein said field-of-view switching rate is greater than said read cycle rate.
 12. The imaging system of claim 1, further comprising: (a) a first set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; (b) a second set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; and (c) a sensor switching arrangement associated with said two-dimensional imaging sensor array, said first and second sets of read capacitors and said optical switching mechanism, said sensor switching arrangement being configured to switch connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said first and second sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view such that said first set of read capacitors accumulate information corresponding to a first of said two fields of view and said second set of read capacitors accumulate information corresponding to a second of said two fields of view.
 13. The imaging system of claim 12, wherein said sensor switching arrangement switches said connections at a field-of-view switching rate, the imaging system further comprising a read arrangement for reading accumulated information from said first and second sets of read capacitors at a read cycle rate, wherein said field-of-view switching rate is greater than said read cycle rate.
 14. The imaging system of claim 13, further comprising: (a) a third set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; and (b) a fourth set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array, wherein said sensor switching arrangement and said read arrangement are further associated with said third and fourth sets of read capacitors, said sensor switching arrangement and said read arrangement being configured such that: (i) during a first half of said read cycle, said sensor switching arrangement switches connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said first and second sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view while said read arrangement reads said third and fourth sets of read capacitors; and (ii) during a second half of said read cycle, said sensor switching arrangement switches connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said third and fourth sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view while said read arrangement reads said first and second sets of read capacitors.
 15. The imaging system of claim 1, further comprising a processor configured for analyzing a sequence of images from said imaging system to determine whether they indicate the presence of a transient event.
 16. An infrared search and tracking (IRST) system for a waterborne vessel, the system comprising: (a) at least one stabilized platform; and (b) an imaging assembly deployed on said at least one stabilized platform, said imaging assembly including a plurality of the imaging systems of claim 1, wherein said imaging systems are deployed in fixed spatial relation such that said fields of view of said plurality of imaging systems together form a substantially contiguous effective field of view spanning substantially 360°.
 17. The IRST system of claim 16, wherein said at least one stabilized platform is implemented as a plurality of said stabilized platforms, and wherein each of said stabilized platforms carries a corresponding subgroup of said plurality of imaging systems, said subgroup providing fields of view which together form a substantially contiguous effective field of view spanning a corresponding given angle, said corresponding given angles together substantially spanning 360°.
 18. The IRST system of claim 16, wherein said at least one stabilized platform is implemented as two stabilized platforms for deployment on opposite sides of the waterborne vessel, and wherein a first of said stabilized platforms carries a first subgroup of said plurality of imaging systems, said first subgroup providing fields of view which together form a substantially contiguous effective field of view spanning at least a first angle, and wherein the second of said stabilized platforms carries a second subgroup of said plurality of imaging systems, said second subgroup providing fields of view which together form a substantially contiguous effective field of view spanning at least 360° minus said first angle, said first and second subgroups thereby together providing an effective field of view substantially spanning 360°.
 19. The IRST system of claim 16, wherein said at least one stabilized platform is stabilized to within a predefined level of precision, the system further comprising a fine stabilization arrangement including: (a) an inertial sensor arrangement associated with said stabilized platform for measuring residual motion of said stabilized platform; and (b) a processing system responsive to said inertial sensor arrangement to process images from said imaging assembly so as to correct said images so as to compensate for said residual motion.
 20. The IRST system of claim 16, wherein each of said fields of view for each imaging system has a spatial resolution of at least two pixels per mille-radian.
 21. The IRST system of claim 16, wherein said substantially contiguous effective field of view spans substantially 360° in azimuth and spans a range of at least about 8° in elevation.
 22. The IRST system of claim 16, further comprising a processing system associated with said imaging assembly and configured to process images from said imaging systems according to a set of target detection criteria to identify suspected targets.
 23. The IRST system of claim 22, wherein said target detection criteria include substantially continuous detection of a suspected target for a period in excess of about half a second.
 24. The IRST system of claim 22, further comprising a gimbaled narrow field-of-view infrared imaging sensor having a field of view not greater than about 3°, said gimbaled narrow field-of-view infrared imaging sensor being associated with said processing system for directing towards suspected targets for evaluation of the suspected targets.
 25. The IRST system of claim 16, wherein said optical switching mechanism is configured to switch said optical axis of the imaging system in two non-parallel switching planes such that each imaging system generates images of at least four fields of view forming a two-dimensional array of fields of view.
 26. An infrared search and tracking (IRST) system comprising an imaging assembly including a plurality of the imaging systems of claim 1, wherein said imaging systems are deployed in fixed spatial relation such that said fields of view of said plurality of imaging systems together form a substantially contiguous effective field of view spanning at least about 40°.
 27. A method for acquiring images of two fields of view using a single two-dimensional imaging sensor array, the method comprising: (a) providing an optical arrangement including an optical switching mechanism for alternately directing an image of the two fields of view onto the imaging sensor array; (b) providing for each sensor of the imaging sensor array at least one pair of read capacitors and a sensor switching arrangement for switching a connection from each of said sensors between the corresponding pair of read capacitors; (c) synchronously operating said optical switching mechanism and said sensor switching arrangement at a field-of-view switching rate such that a first of said pair of read capacitors accumulates information corresponding to a pixel of a first of the fields of view and a second of said pair of read capacitors accumulates information corresponding to a pixel of a second of the fields of view; and (d) reading accumulated information from said pairs of read capacitors at a read cycle rate so as to acquire a sequence of images of each of the two fields of view, wherein said field-of-view switching rate is greater than said read cycle rate.
 28. The method of claim 27, wherein said field-of-view switching rate is at least four times said read cycle rate.
 29. The method of claim 27, further comprising analyzing said sequence of images to determine whether they indicate the presence of a transient event. 